Path protection is a technique employed in fiber-optic networks to protect against inevitable failures at the physical layer that might affect the services offered to end customers. This type of protection provides a way of maintaining service in spite of fiber cuts, dirty connectors, deteriorating optical losses or other degradations that cause the optical power level to drop below acceptable levels. A simple form of path protection is shown in FIG. 1.
In this scheme, a redundant “protection path” is provided in addition to the primary fiber path. A 50% optical coupler distributes the optical signal power equally into both paths, and a single-pole-double-throw (SPDT) optical switch is used to select the path with lower optical losses. Two photodiodes are used to measure the power in the two paths (IN1 and IN2 in FIG. 1) and switching decision are based on these power levels. The photodiodes are often incorporated within the body of the SPDT switch.
A serious disadvantage of this simple protection switching technique is that the 50% optical coupler results in an excess optical loss of 3 dB, reflecting the fact that half of the optical power (namely the power in the unused path) is wasted. It might seem that, in a bi-directional network where there are optical signals present in the reverse direction also, it might be possible to replace the 50% optical coupler in FIG. 1 with a second SPDT switch as shown in FIG. 2.
In this technique, there are two SPDT optical switches used so that the excess 3 dB loss of the 50% coupler is avoided. In order for the switch labeled as SPDT1 to make switching decision, there are also two photodiodes that are used to measure the optical power level in the upstream direction in the primary and protection paths (US1 and US2, respectively).
The major disadvantage of the protection technique shown in FIG. 2 is that using a cascade of two switches reduces loss but almost invariably results in instability that causes indefinitely long toggling of the switches from one state to another (even if the switches themselves are ideal with no chatter). Such rapid and long-lasting toggling of the optical switches can soon result in catastrophic failure of the switches.
A more stable protection switching technique where the first SPDT1 switch is replaced by a cross-bar (Xbar) switch is shown in FIG. 3.
The cross-bar switch is a 2×2 optical switch that has two states: (1) a BAR state in which inputs 1 and 2 are coupled directly to outputs 1 and 2, respectively, and (2) a CROSS state in which inputs 1 and 2 are coupled to outputs 2 and 1, respectively. FIG. 3 shows the cross-bar switch in a BAR state. An optical coupler is used to ensure that under normal conditions there is light propagating down both the primary and protection paths. The coupler can be asymmetric so that most of the light normally goes through the primary path. FIG. 3 shows the case where the optical coupler distributes the light in a 95% versus 5% split. The purpose of the cross-bar switch is to ensure that most of the downstream light goes though the path that has lower loss.
Since the coupler shown in FIG. 3 is not a 50% coupler, this protection technique has as much as 2.5 dB lower loss than the simple protection scheme shown in FIG. 1. This is a very important advantage of this protection technique.